The present invention relates to a control rod for controlling output of a BWR (boiling water reactor), a unit for a control rod and a method of producing control rods.
Examples of this kind of control rod for BWR, having blades arranged in an approximately cruciform shape are disclosed in JP A 5-2092 and JP A 9-61576.
Those conventional control rods is each have such a construction in which a handle is mounted on an axially upper portion of a tie rod of approximately cruciform cross-section and a lower support member (or a falling speed limiter, hereunder referred to in the same manner) is mounted on an axially lower portion of the tie rod, an upper hafnium formed in a tube of approximately cruciform cross-section is fixed to the handle and a lower hafnium formed in a tube in the same manner is fixed to the lower support member, further, four sheaths each containing therein the upper and lower hafnium as reactivity control material are fixed to the handle, the lower support member and approximately cruciform end portions of the tie rod.
At this time, any production process of the above-mentioned control rod is not disclosed in the above-mentioned Japanese Laid-open Patent Applications, however, usually the following process is taken.
That is, first, a handle that an upper tongue-shaped portion extending downward and a handle body are formed in one block is mounted on an axially upper portion of a tie rod. A lower support member that a lower tongue-shaped portion extending upward and a lower support member body are formed in one block is fixed to an axially lower portion of the tie rod. Thereby, an assembly (it also is called a frame) of the handle, tie rod and lower support member is completed.
After the upper tongue-shaped portion of the handle is inserted into an upper hafnium elliptic tube from an upper side thereof, the inserted upper tongue-shaped portion and the upper hafnium elliptic tube are connected by a fixing pin passing through both of them. On the other hand, about the lower tongue-shaped portion, also, in the same manner as above it is inserted into an lower hafnium elliptic tube from a downside thereof, and the lower tongue-shaped portion and the lower hafnium elliptic tube are connected by a fixing pin.
As mentioned above, the upper and lower hafnium elliptic tubes are fixed, and then those upper and lower. hafnium elliptic tubes are taken in, in turn, from a U-shaped opening side of a stainless steel sheath of U-shaped cross-section so that all the upper and lower hafnium elliptic tubes are arranged and contained inside the sheath.
After that, an end portion of the sheath U-shaped opening side is welded and fixed to a cruciform end surface corresponding to a tie rod of cruciform cross-section (for example, welded at several positions in the length direction), finally, upper and lower end portions of the sheath are welded and fixed to a lower portion of the handle and to an upper portion of the lower support member.
However, the following subjects exist in the above-mentioned prior arts.
Generally, as for control rods for BWR provided with blades arranged in an approximately cruciform shape, there are a plurality of kinds of control rods according to the specification and performance, etc. of BWRs, and the control rods have different shapes of reactivity control material and fixing structures thereof according to the kinds. In the above-mentioned JP A 5-2092 and JP A 9-61576, the hafnium tube is used as the reactivity control material (neutron absorber), as mentioned above. And, an upper tongue-shaped portion provided in the handle in one block is connected to the upper hafnium tube by a pin and a lower tongue-shaped portion provided in the lower support member in one block is connected to the lower hafnium tube by a pin, whereby the hafnilum tubes are fixed from both upper and lower sides.
On the other hand, as another type of control rod, a construction using a neutron absorbing rod that boron carbide (B4C) is filled in a stainless steel cladding for example is proposed, as disclosed in JP A 11-94975. In this case, any engaging or fixing construction of each neutron absorbing rod and the handle and lower support member sides.is not provided, the each neutron absorbing rod is only mounted on the lower support member by the weight of the rod itself in the sheath.
As the other type of control rod, there is already proposed a construction in which a hafnium solid rod (approximately circular rod shape) is used as reactivity control material, and a convex structure provided at an upper portion of each hafnium rod and a curtain rail-shaped concave structure are engaged thereby to support the hafnium rod in a sheath. In this case, any engaging or fixing structure of each hafnium rod and a lower support member side is not provided.
The control rod using the hafnium tube disclosed in the above-mentioned JP A 5-2092 and JP A 9-61576 is very similar to the control rod using the B4C-containing neutron absorbing rod disclosed in the above-mentioned JP A 11-94975 and the control rod in which hafnium solid rods are engaged with and supported by curtain rail-shaped constructions in a point of a basic construction in which a handle and a lower support member are mounted on upper and lower portions of a tie rod of cruciform cross-section, a sheath containing therein reactivity control material is arranged in a space between the handle and the lower support member, and different therefrom only in a construction on the reactivity control side. Therefore, if the similar constructions can be made into common parts, the common parts can be applied to each of the above-mentioned three kinds of control rods.
However, in the above mentioned JP A 5-2029 and JP A 9-61576, such a point is not considered. That is, when the handle, tie rod and lower support member are assembled in advance as an assembly, the upper tongue-shaped portion that the upper hafnium tube is to be mounted to is already formed at the lower portion of the handle, and the lower tongue-shaped portion that the lower hafnium tube is to be mounted to is already formed at the upper portion of the lower support member. Therefore, it was impossible to apply for the control rod using the above-mentioned B4C-containing neutron absorbing rod and for the control rod engaging and supporting hafnium solid rods, both types of which do not use fixing structures by such tongue-shaped portions.
As a result, irrespective of existence of relatively many parts of similar structure, it is necessary to produce by process and equipment different from the above-mentioned two other types of control rods, whereby the process and production equipment were increased and the production efficiency (productivity) was lowered. Further, since control rods which are not so greatly different from each other were produced, transported and stored for each type, matters increasing disadvantages on transportation and requiring a wide storage space were noticed.
An object of the present invention is to provide a structure in which parts are made common to a plurality of types and the production process and equipment are reduced whereby the productivity can be improved, in a control rod of a BWR provided with blades arranged in an approximately cruciform shape.
(1) In the present invention, a first unit which is constructed by a tie rod, a handle mounted on an axially upper portion of the tie rod and a lower support member or falling speed limiter mounted on an axially lower portion of the tie rod, and a second unit which is constructed by a sheath containing therein reactivity control material, an upper end plate mounted on an axially upper portion of the sheath and a lower end plate mounted on an axially lower portion of the sheath are provided, an upper portion of the above-mentioned upper end plate is fixed to the above-mentioned handle, a lower portion of the above-mentioned lower end plate is fixed to the above-mentioned lower support member or falling speed limiter, and the present invention is constructed as a joined body of the above-mentioned first unit and the above-mentioned second unit.
In the present invention, a lower portion side region of the handle, on which is reactivity control material is mounted in a conventional construction is separated as the upper end plate to be a separate member, and in the same manner, an upper portion side region of the lower support member (or falling speed limiter, hereunder referred in the same manner) is separated as the lower end plate to be a separate member. The handle of which the lower portion side region is separated and the lower support member of which the upper portion side region is separated are mounted on upper and lower portions of the tie rod, respectively, thereby to make the first unit, while the upper end plate separated from the handle and the lower end plate separated from the lower support plate are mounted on the sheath containing therein reactivity control material, as mentioned above, thereby to make the second unit.
Thereby, for the control rod using the upper hafnium tube and lower hafnium tube, the upper and lower tongue portions are fixed to the upper and lower end plates, respectively, and the upper and lower hafnium tubes are connected to them by pins and covered with the sheath, whereby the second unit for specific use is prepared. As can be seen in FIG. 2, the hafnium tubes and the sheath can be, for example, made of a hollow tube with substantially straight sections having U-shaped end sections at both lateral ends. For the control rod using the neutron absorbing rods containing therein B4C, the neutron absorbing rods are disposed on the lower end plate and covered with the sheath, whereby the second unit for specific use is prepared. For the control rod using hafnium solid rods, the second unit for specific use formed by engaging a curtain rail-shaped concave portion formed at a lower portion of the upper end plate with a convex portion formed at an upper portion of each hafnium rod and covered with the sheath is prepared; whereby the first unit of an assembly of the handle, the lower support member and the tie rod can be commonly used for the above-mentioned three types of control rods (having them in common).
Therefore, the process and equipment can be reduced and the productivity can be improved compared with conventional process and equipment in which different process and equipment were necessary for each type of control rod. Further, for the first units which are made common, it is possible to store in the same place through the same transferring route. And only the second units which are different from each other need to be stored at different places. Therefore, there is also such an effect that the storage spaces can be reduced without suffering from disadvantages on transportation.
(2) In the above-mentioned item (1), preferably, the above-mentioned sheath is made of a hollow tube of substantially straight sections having U-shaped end sections at both lateral ends.
Thereby, it is possible to form a closed space containing therein the reactivity control material by only the sheath, so that it is unnecessary to weld and fix a U-shaped open end of the sheath to a cruciform end surface of a tie rod of cruciform cross-section type, as in a conventional construction. That is, it is possible to omit complicated welding of plural positions in the length direction of the tie rod which is long and several meters in whole length, so that the process can be simplified and the reliability can be raised as well.
(3) In the above items (1) or (2), preferably, the above-mentioned reactivity control material is made of at least one of a hafnium tube of substantially straight sections having U-shaped end sections at both lateral ends, a hafnium rod and a neutron absorbing rod containing boron carbide.
(4) A unit for a control rod according to the present invention comprises a sheath made of an elliptic tube of approximately cruciform cross-section and containing therein reactivity control material, an upper end plate mounted on an axially upper portion of the above-mentioned heath, and a lower end plate mounted on an axially lower portion of the above-mentioned sheath.
(5) A production process of a control rod according to the present invention comprises the steps of mounting a handle on an axially upper portion of a tie rod, and a lower support member or falling speed limiter on an axially lower portion of the tie rod, thereby to form a first unit; mounting an upper end plate on an axially upper portion of a sheath containing therein reactivity control material, and a lower end plate on an axially lower portion of the sheath, thereby to form a second unit; and then fixing an upper portion of the above-mentioned upper end plate to the above-mentioned handle, and a lower portion of the above-mentioned lower end plate to the above-mentioned lower support member or falling speed limiter, thereby to join the above-mentioned first unit and the above-mentioned second unit.